1. Field of the Invention
This invention relates generally to maximizing radio frequency transmission efficiency while minimizing spurious harmonic emission in a wireless communication device transmitter. More particularly, the invention relates to a wireless transmitter architecture that incorporates a synchronous oscillator to modify a transmit signal in a translation loop upconverter.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
As these mobile communication systems have been developed and deployed, many different standards have evolved, to which these systems must conform. For example, in the United States, many portable communications systems comply with the IS-136 standard, which requires the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme can be 8-quadrature phase shift keying (8QPSK), offset π/4 differential quadrature phase shift keying (π/4-DQPSK) or variations thereof, and the access format is TDMA.
In Europe, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift keying (GMSK) modulation scheme in a narrow band TDMA access environment, which uses a constant envelope modulation methodology.
Furthermore, in a typical GSM mobile communication system using narrow band TDMA technology, a GMSK modulation scheme supplies a low noise phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator. In such an arrangement, a highly efficient, non-linear power amplifier can be used thus allowing efficient modulation of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power-amplifier, is minimized. Further, the output in a GSM transceiver is a constant envelope (i.e., a non time-varying amplitude) modulation signal.
The dominant transmitter architecture for GSM compliant devices is known to those having ordinary skill in the art as an “upconversion loop,” or a “translation loop.” Typically, the translation loop is supplied with a modulated intermediate frequency (IF) signal from an in-phase (I)/quadrature (Q) (IQ) modulator.
Unfortunately, due to imperfect gain and phase imbalance, an IQ modulator emits undesired signals, in the form of local oscillator (LO) signal leakage and opposite sideband components. Further, an IQ modulator emits IF carrier harmonics that are modulated by the fundamental I and Q signals supplied by the IQ modulator.
An IQ modulator also emits harmonics at the third and fifth multiples (sometimes called “overtones” or nth order harmonics) of the IF carrier frequency. These third and fifth order harmonics cause spurious tones to appear in the radio frequency (RF) output signal in the vicinity of the carrier center frequency after being upconverted to the RF frequency by the translation loop. The unwanted spurious tones add to the phase error of the transmit signal and increase undesired spectral output in the vicinity of the carrier frequency.
The presence of the undesired spurious tones in the vicinity of the carrier frequency can cause the output of a GSM transmitter to violate the strict modulation spectrum standard present in GSM. This standard is commonly referred to in the industry as a “modulation mask” specification.
Reducing the LO signal leakage and the opposite sideband components is typically accomplished by calibrating the IQ modulator during the final stages of production of the portable transceiver. Unfortunately, this does not completely resolve the problems in the IQ modulator and could increase the cost of the portable transceiver.
The spurious tones caused by the third and fifth order harmonics can be reduced before reaching the translation loop by incorporating an IF filter at the output of the IQ modulator. Unfortunately, such a filter requires many passive electrical components and, due to the complexity of the filter and associated manufacturing difficulties, the filter is generally placed on a different chip (i.e., on a substrate other than the substrate on which the transmit circuitry is fabricated). Further, such a filter achieves only moderate performance improvement, and the resulting transmit circuitry may still violate the strict modulation spectrum standard present in GSM.
Therefore it would be desirable to provide a transmit architecture for a portable transceiver that is economical to manufacture and that exhibits a superior output signal characteristic.